This invention relates to hydraulic valves of the type which are intended to direct a hydraulic fluid as well as control the size of a flow to a flow consuming device comprising load objects in the form of hydraulic actuators and/or other types of hydraulic motors.
The above mentioned hydraulic valves may be of the two main types: open centre valve or closed centre valve.
The former type of valve is intended to work in combination with a hydraulic pump having constant displacement and being arranged to let the entire pump flow pass through the valve unrestrictedly as the valve occupies its neutral or inactivated position. As the valve is activated the desired flow is directed to the load object, usually by bypass control.
The latter type of valve is intended to work in combination with a variable displacement pump and an automatically operating shunt. When inactivated, the directional valve itself is closed as regards the pump flow. The actual type of valve is used either in a system with constant pump pressure, then it is called constant pressure valve, or in a system in which the pump pressure corresponds to the heaviest load in each moment. Then the valve is referred to as load sensing. The invention is mainly related to the last type of valve, also called LS valve, but to some extent it is related to constant pressure valves and constant flow valves.
Accelerations of inertial loads at for instance the slew function in an excavator, result in increases in the load pressure which are proportional to the magnitude of the acceleration. If the acceleration is fast enough toward a desired speed, the result usually is that when the desired speed is reached the hydraulic motor suddenly operates as a pump driven by the inertial load. The continuous condition is disturbed in that the motor momentarily consumes more fluid than what is delivered from the pump.
Accordingly, there will be a shortage of fluid resulting in the pressure falling towards zero.
Consequently, the inertial load is retarded, and in the next sequence the load is re-accelerated by the pump flow, which results in a pressure increase. Due to this and to the elasticity inherent in the system, the result is an oscillation of a low frequency and rather a big amplitude which influences negatively the controllability and makes precision movements more difficult. This problem is most significant at LS valves which have a poor internal damping. At slow accelerations, there are mostly aperiodic oscillations, and the abovementioned problems do not occur.
One way of improving this condition is to adapt the valve spool restrictions as regards the flow to and from the motor such that a certain pressure above the load pressure is maintained. By this arrangement it is possible to improve the stiffness of the motor, and, thereby reduce the oscillations. Another way of doing this is to install a double overcenter valve at the main connections of the motor. Then, the motor can not "run ahead" of the flow and cause large pressure variations in the system. Unfortunately, it is not possible to have such a valve function to operate completely satisfactorily without causing large pressure drop losses in the power circuit as well as damping on the activation side. More effective methods may be used though, methods that can be used with less losses and a safer operation.
In the European Patent Publication No. EP 0066717 there is shown a solution to the problem of how to start and stop softly heavy inertial loads. The motor pressure is arranged to act upon auxiliary pistons located in the inlet part of the valve spool and which counteract the activation signal. Thereby, the valve spool displacement is counteracted and a damping action is obtained. The return force acting on the spool acts in the same instant the pressure distortion occurs, which means that the latter is damped without any phase shift.
Another solution to the problem and which gives a softer damping than the above mentioned is described below.